Make remote-test-client and remote-test-server compatible with windows
`compiletest` and `remote-test-client`:
The command line for `remote-test-client` was changed slightly to allow cross-platform compatible paths. The old way of supplying the support libs was by joining their paths with the executable path
with `:`. This caused Windows-style paths to be split after the directory letter. Now, the number of support libs is provided as a parameter as well, and the support lib paths are split off from the regular args in the client.
`remote-test-server`:
- Marked Unix-only parts as such and implemented Windows alternatives
- On Windows `LD_LIBRARY_PATH` doesn't exist. Libraries are loaded from `PATH` though, so that's the way around it.
- Tiny cleanup: `Command::args`/`envs` instead of manually looping over them
- The temp path for Windows has to be set via environment variable, since there isn't a global temp directory that would work on every machine (as a static string)
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